The ipv6only field of the ngx_listening_t structure was changed to accept only 0 or 1 to mean turn IPV6_V6ONLY off or on, respectively. The old semantics of 0 (leave IPV6_V6ONLY at OS default) is no longer supported.
3rd party modules that create their own listening sockets might need fixing. At least the following modules (authors Cc:ed) are known to be affected by this change: rtmp, tcp proxy.
Please see the following commit for details: http://trac.nginx.org/nginx/changeset/4772/nginx